Traditional precious metals (PM) refining processes involving a series of precipitation-dissolution steps are considered inefficient by today's standards in terms of the degree of separation achieved, the yields obtained and the complexity of the operation. Thus, a considerable amount of research and development work has been conducted in the last ten years or so with the purpose of developing solvent extraction (SX) based processes to overcome the problems related to traditional practice. Currently there are three such processes, one developed by INCO and operated at their Acton refinery in England, (Barnes, Edwards, 1982), one operated by Lonrho in South Africa (Edwards, 1979), and the third developed by Matthey Rustenburg Refiners (MRR) for which a plant was constructed in England (Reavill, Charlesworth, 1980). The three processes were compared and discussed recently by Flett (1982). The main advantages for the SX based processes are: (a) less lockup of valuable metal due to reduced overall processing time, (b) improved primary yields, (c) reduced process recycles, (d) flexibility and versatility, and (e) capability for increased process control.
The processes adopted by INCO and MRR and to some extent by Lonrho in their refineries are the selective extraction-separation of the various precious metals in successive stages using different extractants.
In current commercial refining processes, gold (AuCl.sub.4.sup.-) is extracted first from the primary precious metals with the use of solvating reagents such as dibutyl carbitol by INCO or methyl isobutyl ketone (MIBK) by MRR. After Au removal, anion exchanging reagents can be used for the co-extraction of Pt and Pd as practiced by Lonrho. However, selective stripping does not appear to be easily achievable (Cleare, Charlesworth, Bryson, 1979). Therefore, further chemical separation or use of another selective extractant is required for ultimate separation of Pd and Pt. Selective extraction of Pd is achieved with the use of neutral alkyl sulphides (INCO), or with the use of chelating agents such as hydroxyoximes (MRR). Both processes however are characterized by very slow kinetics. Platinum is finally recovered with anion exchange extractants such as tributyl phosphate (TBP) by INCO or amines (MRR).
Carbitols (trademark) are a group of mono and dialkyl ethers of diethylene glycol and their derivatives.
After extraction and stripping of each of the precious metals, salt precipitation, i.e., Pd(NH.sub.3).sub.2 Cl.sub.2 and (NH.sub.4).sub.2 PtCl.sub.6 followed by salt calcination are usually used to effect final metal recovery. Despite their advantages over the classical precipitation route the present solvent extraction processes have some unattractive features. These are: (a) long contact times (Pd, several hours), (b) need for interstage adjustments of feed composition, (c) use of different extractants and (d) multi-step recovery circuits.
Recently, comparative studies (Agarwal, Klumpar, 1979) on both routes, (a) selective extraction and (b) selective stripping for the separation of different base metals have concluded that the latter route appears to be economically more attractive than the former for base metals. We consider a selective stripping route might prove especially suitable for precious metals refining due to the complex solution chemistry involved.
Precious metals, in addition to their complex solution chemistry which enables us to apply solvent extraction processes for their separation, are also well known for their thermodynamic amenability to reduction to their metallic forms by the use of hydrogen gas (Findlay, 1983). It appeared to us that these two techniques, solvent extraction and hydrogen reduction, could be applied to the direct recovery of precious metals, and this is the basis of our invention.
In recent years the concept of the integration of solvent extraction and hydrogen reduction was introduced, and was applied for the production of some base metal powders (Burkin, 1973; Demopoulos, 1981). This operation, also called pressure hydrogen stripping, involves direct metal precipitation from the loaded organic phase using hydrogen in an autoclave. A modified version of this technique is the so called hydrolytic stripping (Thorsen, Monhemius, 1979) where instead of hydrogen, water at elevated temperatures is used. Very recently, the successful precipitation of Au with hydrogen from loaded Carbitol organic solvents was also reported (Li, Demopoulos, Distin, 1983).